Since a condensation-polymerization type polyimide becomes insoluble in an organic solvent with an increase of the degree of polymerization, when this polymer is used as a matrix resin of a composite material, a process is adopted in which a so-called amide acid prepolymer before dehydration ring closure is dissolved in a high-boiling-point solvent such as N-methylpyrrolidone (NMP) or dimethylformamide (DMF) and a reinforcing fiber is impregnated with this prepolymer solution. According to this process, however, it is difficult to remove water of condensation formed at the ring closure at the molding step or the high-boiling-point solvent, whereby these volatile components are left in the molded product and voids are formed in the molded article, resulting in a reduction of the physical properties of the molded article.
An addition-curing type polyimide has been developed as a material for solving the above-mentioned problem. The addition-curing type polyimide is an oligomer having a molecular weight of about 1,500 and containing addition-reactive nadic acid at terminals, and this oligomer is a heat-curable resin which is converted through an amic acid prepolymer to a polyimide polymer, which is crosslinked by the ring-opening addition reaction of terminal nadic acid. This addition-curing type polyimide, however, has a problem in that it is soluble only in a high-boiling-point solvent such as NMP or DMF.
As the result of investigations made with a view to improving this solubility, a PMR type polyimide (in-situ polymerization of monomeric reactants), which is formed by esterification of a carboxylic anhydride type monomer with an alcohol followed by dissolving the resulting ester in an alcohol as the solvent, has been developed by TRM Systems Co., U.S.A. [J. Appl. Polym. Sci., 16, (1972), 905]. In contrast to the conventional polyimide wherein a varnish is prepared by dissolving the amic acid prepolymer in a high-boiling-point solvent, this PMR type polyimide is characterized in that a varnish is prepared by dissolving monomers in a low-boiling-point solvent. For example, in the case of PMR-15, which is a typical resin of the PMR type polyimide, a varnish is prepared by dissolving dimethyl 3,3',4,4'-benzophenonetetracarboxylate (BTDE), monomethyl nadate (NE) and 4,4'-diaminodiphenylmethane (DDM) as the monomers directly in methanol, and a prepreg is prepared by using this varnish according to the wet method. NE, however, has a higher reactivity with a diamine such as DDM, compared with an aromatic tetracarboxylic acid diester such as BTDE, and therefore, during storage of the varnish, the reaction between NE and DDM is preferentially advanced and bisnadimide (BNI) having a low molecular weight is formed [ACS Organic Coatings and Plastics, 40, (1979), 935 and J. Appl. Polym. Sci., 27, (1982), 4295]. Since the monomer composition in the prepared resin solution is thus changed during the storage, it is very difficult to prepare a prepreg having a stable quality.
If BNI is formed in a large quantity, the remaining monomers form a high-molecular-weight oligomer, and therefore, the molecular weight distribution of the resin as a whole is changed, resulting in a degradation of the moldability, for example, a reduction of the flowability during the molding operation. Furthermore, in the thus-obtained composite material, the mechanical properties such as interlaminar shear strength and flexural strength are reduced, and adverse influences are imposed on the heat resistance, for example, a lowering of the heat decomposition temperature occurs, [National SAMPE Symposium, 26, (1981), 89].